1 1 12 VISION. 



the departures from Newton's law, by supposing that visual purple, with 

 feeble decomposition, gives rise to a colourless sensation, but that with 

 stronger decomposition the visual yellow formed is the basis of the blue 

 component of the Young-Helmholtz theory. The former supposition 

 has already been considered, and the latter was based on a correspond- 

 ence between the curve of absorption of visual yellow with the curve of 

 the elementary blue sensation calculated by Konig and Dieterici, and on 

 the supposed blue-blindness of the fovea. 



Konig places the red and green substances in the pigment epithelium, 

 and regards the cones as dioptric mechanisms. It would follow from 

 Konig's hypothesis that in red-green-blindness the fovea should have 

 only monochromatic vision. 



In 1884 an attempt was made by Holmgren l to prove the truth of 

 the three-colour theory by stimulation of minute retinal points. He 

 found that a minute yellow point of light appeared red at some points 

 of the retina, green at others, and concluded that this depended on the 

 stimulation of elements for red and green respectively. Hering, 2 how- 

 ever, showed that Holmgren's results were due to his use of impure light, 

 and did not occur with true homogeneous yellow light. Isaachsen 8 also, 

 working under Konig, failed to confirm Holmgren's results. Holmgren, 4 

 later, repeated his experiment with homogeneous spectral light, and with 

 a number of observers, some of whom saw the phenomenon, others with 

 difficulty, or not at all. He obtained very indefinite results with blue light. 



If the recent theory of the function of visual purple is correct, the 

 Young-Helmholtz theory will not be weakened ; in fact, the acceptance of 

 the former will to some extent assist the latter theory, for it relieves the 

 latter from the difficult necessity of explaining total colour-blindness and 

 Purkinje's phenomenon. If these depend upon the visual purple in the 

 rods, which function independently of the chromatic apparatus, the cones 

 may be the basis of the colour vision. But though visual purple may 

 relieve the Young-Helmholtz theory of some of its difficulties, those 

 connected with the explanation of after-images and contrast, and the 

 subjective sensations of the colour-blind, still remain. 



Hering's theory.— The starting-point of Hering's theory was the 

 failure of Helmholtz to give a physiological explanation of contrast. 

 Hering also objected to some of the fundamental principles of Young's 

 theory. Any theory of sensation must have a physical, a physiological, 

 and a psychological aspect ; and Hering objected to the psychological 

 aspect of the three-colour theory. In 1865, Mach 5 had pointed out that 

 the black-white series of sensations differed wholly in quality from the 

 chromatic series, and that, to be consistent with psycho-physical 

 parallelism, the two series should have a separate physiological basis. 

 Consciousness is totally unable to analyse the sensation white into any 

 coloured components. He further pointed out that in the series 

 of colour sensations there are four which appear simple and unde- 

 composable, namely, red, yellow, green, and blue, — and that each 

 of these colours should have a separate physiological basis. Hering 

 adopted the same attitude, and pointed out that two of the original three 

 primary colour sensations of Helmholtz were manifestly complex ; that 



1 Cong, piriod-internat. d. sc. med. C. r., Copenhagen, 1884, tome i. p. 93. 



2 Arch./, d. ges. Physiol., Bonn, 1887, Bd. xl. S. 1. 



3 Ibid., 1888, Bd. xliii. S. 289. 



4 Skandin. Arch./. Physiol.. Leipzig, 1889, Bd. i. S. 152 ; and Bd. iii. S. 253. 



5 Sitzungsb. d. k. Akad. d. Wissensch., Wien, 1865, Bd. Iii. Abth. 2, S. 320. 



